Method for selectively producing acetic acid through the catalytic oxidation of ethane

ABSTRACT

The invention relates to a method for selectively producing acetic acid from a gaseous feedstock of ethane, ethylene or mixtures thereof and oxygen at a high temperature. Said gaseous feedstock is brought together with a catalyst containing the elements Mo, Pd, X and Y in the gram-atomic ratios a:b:c:d in combination with oxygen: Mo a Pd b X c Y d  (I). Symbols X and Y have the following meaning: X represents one or several of the elements chosen from the group Cr, Mn, Nb, Ta, Ti, V, Te and W; Y represents one or several of the elements chosen from the group B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Nb, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl and U, the indices a, b, c, d and x represent the gram-atomic ratios of the corresponding elements: a=1, b=0.0001 to 0.01, c=0.4 to 1 and d=0.005 to 1. The space-time yield in the oxidation to acetic acid using the inventive method is &gt;150 kg/hm 3 . The selectivity of the oxidation reaction of ethane and/or ethylene to acetic acid is especially≧70 mol %.

This application is a 371 of PCT/EP98/06414 filed Oct. 9, 1998.

The present invention relates to a process for the selective preparationof acetic acid by catalytic gas-phase oxidation of ethane and/orethylene in the presence of a palladium-containing catalyst.

The oxidative dehydrogenation of ethane to ethylene in the gas phase attemperatures of >500° C. is known, for example, from U.S. Pat. No.4,250,346, U.S. Pat. No. 4,524,236 and U.S. Pat. No. 4,568,790.

Thus, U.S. Pat. No. 4,250,346 describes the use of a catalystcomposition comprising the elements molybdenum, X and Y in a ratio ofa:b:c for converting ethane into ethylene, where X is Cr, Mn, Nb, Ta,Ti, V and/or W and Y is Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si,Sn, TI and/or U and a is 1, b is from 0.05 to 1 and c is from 0 to 2.The total value of c for Co, Ni and/or Fe has to be less than 0.5. Thereaction is preferably carried out in the presence of added water. Thedisclosed catalysts can likewise be used for the oxidation of ethane toacetic acid, with the efficiency of the conversion to acetic acid beingabout 18% at an ethane conversion of 7.5%.

The abovementioned documents are concerned mainly with the preparationof ethylene, less with the targeted preparation of acetic acid.

In contrast, EP-B-0 294 845 describes a process for the selectivepreparation of acetic acid from ethane, ethylene or mixtures thereofusing oxygen in the presence of a catalyst mixture comprising at leastA.) a calcined catalyst of the formula Mo_(x)V_(y) or Mo_(x)V_(y)Z_(y),where Z may be one or more of the metals Li, Na, Be, Mg, Ca, Sr, Ba, Zn,Cd, Hg, Sc, Y, La, Ce, Al, TI, Ti, Zr, Hf, Pb, Nb, Ta, As, Sb, Bi, Cr,W, U, Te, Fe, Co and Ni and x is from 0.5 to 0.9, y is from 0.1 to 0.4and z is from 0.001 to 1, and B.) an ethylene hydration catalyst and/orethylene oxidation catalyst. The second catalyst component B is, inparticular, a molecular sieve catalyst or a palladium-containingoxidation catalyst. When using the catalyst mixture described andpassing a gas mixture comprising ethane, oxygen, nitrogen and watervapor through the reactor containing the catalyst, the maximumselectivity is 27% at an ethane conversion of 7%. According to EP 0 294845, the high conversions of ethane are achieved only using the catalystmixture described, but not using a single catalyst comprising thecomponents A and B.

A further process for preparing a product comprising ethylene and/oracetic acid is described in EP-B-0 407 091. Here, ethane and/or ethyleneand a gas comprising molecular oxygen is brought into contact with acatalyst composition comprising the elements A, X and Y at elevatedtemperature. A is Mo_(d)Re_(e)W_(f), X is Cr, Mn, Nb, Ta, Ti, V and/or Wand Y is Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si, Sn, TI and/or U.The maximum selectivities which can be achieved when using the catalystdescribed in the oxidation of ethane to acetic acid are 78%. Furtherby-products formed are carbon dioxide, carbon monoxide and ethylene.

The German Patent Application P 19630832.1, which is not a priorpublication, describes a process for the selective preparation of aceticacid from a gaseous feed comprising ethane, ethylene or mixtures thereofplus oxygen at elevated temperature. The feed is brought into contactwith a catalyst comprising the elements Mo, Pd, X and Y in combinationwith oxygen.

Here, X is one or more elements selected from the group consisting ofCr, Mn, Nb, Ta, Ti, V, Te and W and Y is one or more elements selectedfrom the group consisting of B, Al, Ga, In Pt, Zn, Cd, Bi, Ce, Co, Rh,Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Zr, Hf, Ni, P,Pb, Sb, Si, Sn, TI and U.

The gram atom ratios of the corresponding elements are given as follows:

a(Mo)=1; b(Pd)>0; c(X)>0; and d(Y)=0-2.

The catalysts described in the abovementioned application give a maximumspace-time yield of 149 kg/hm³ at an acetic acid selectivity of >60 mol%. Space-time yields characterize the amount of acetic acid produced perunit time and catalyst volume. Higher space-time yields are desirablesince the size of the reactors and the amount of circulated gas can bereduced thereby.

It is therefore an object of the invention to provide a process whichallows ethane and/or ethylene to be oxidized to acetic acid in a simpleand targeted manner and at high selectivity and space-time yield underreaction conditions which are as mild as possible.

It has surprisingly been found that it is possible to oxidize ethaneand/or ethylene to acetic acid under relatively mild conditions in asimple manner at high selectivity and excellent space-time yields whenusing a catalyst comprising the elements molybdenum and palladium andone or more elements selected from the group consisting of chromium,manganese, niobium, tantalum, titanium, vanadium, tellurium and/ortungsten.

The present invention accordingly provides a process for the selectivepreparation of acetic acid from a gaseous feed comprising ethane,ethylene or mixtures thereof plus oxygen at elevated temperature,wherein the gaseous feed is brought into contact with a catalystcomprising the elements Mo, Pd, X and Y in the gram atom ratios a:b:c:din combination with oxygen

MO_(a)Pd_(b)X_(c)Y_(d)  (I)

where the symbols X and Y have the following meanings:

X is one or more elements selected from the group consisting of: Cr, Mn,Ta, Ti, V, Te and W, in particular V and W;

Y is one or more elements selected from the group consisting of: B, Al,Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Cu, Rh, Ir, Au, Ag, Fe, Ru, Os, K, Rb,Cs, Mg, Ca, Sr, Ba, Nb, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, TI and U, inparticular Nb, Ca, Sb and Li.

The indices a, b, c and d are the gram atom ratios of the correspondingelements, where

a=1,

b=0.0001-0.01,

c=0.4-1 and

d=0.005-1.

If X and Y represent a plurality of different elements, the indices cand d can likewise assume a plurality of different values.

Furthermore, the present invention provides a catalyst for the selectivepreparation of acetic acid comprising the elements Mo, Pd, X and Y inthe gram atom ratios a:b:c:d in combination with oxygen.

The gram atom ratios a:b:c:d are preferably within the following ranges:

a=1;

b=0.0001-0.005;

c=0.5-0.8 and

d=0.01-0.3.

Palladium contents in the catalyst which are above the upper limitindicated promote carbon dioxide formation in the process of theinvention. Furthermore, higher palladium contents are generally alsoavoided because they make the catalyst unnecessarily expensive. On theother hand, palladium contents below the limit indicated favor formationof ethylene.

Apart from the elements molybdenum and palladium, the catalyst usedaccording to the invention preferably further comprises vanadium,niobium, antimony and calcium in combination with oxygen. The gram atomratios a:b:c¹:d¹:d²:d³ of the elements Mo:Pd:V:Nb:Sb:Ca are preferablyas follows:

a (Mo)=1;

b (Pd)=0.0001-0.005, in particular 0.0001-0.001;

c¹ (V)=0.4-1.0;

d¹ (Nb)=0.01-0.2;

d² (Sb)=0.01-0.3;

d³ (Ca)=0.01-0.3.

Examples of such catalyst compositions which are preferably used in theprocess of the invention are:

Mo_(1.00)Pd_(0.0005)V_(0.45)Nb_(0.03)Sb_(0.01)Ca_(0.01)

Mo_(1.00)Pd_(0.0005)V_(0.45)Nb_(0.03)Sb_(0.01)Ca_(0.01)K_(0.015)

Mo_(1.00)Pd_(0.00075)V_(0.45)Nb_(0.03)Sb_(0.01)Ca_(0.01)

Mo_(1.00)Pd_(0.00075)V_(0.55)Nb_(0.03)Sb_(0.01)Ca_(0.01)

Mo_(1.00)Pd_(0.00075)V_(0.45)Nb_(0.06)Sb_(0.01)Ca_(0.01)

Mo_(1.00)Pd_(0.0008)V_(0.55)Nb_(0.06)Sb_(0.01)Ca_(0.01)

Mo_(1.00)Pd_(0.00085)V_(0.55)Nb_(0.06)Sb_(0.01)Ca_(0.01)

Mo_(1.00)Pd_(0.00075)V_(0.55)Nb_(0.09)Sb_(0.01)Ca_(0.01)

Mo_(1.00)Pd_(0.0008)V_(0.50)Nb_(0.15)Te_(0.01)Ca_(0.01)

Mo_(1.00)Pd_(0.00075)V_(0.50)Nb_(0.09)W_(0.01)Pd_(0.0003)

The catalysts used according to the invention can be prepared byconventional methods. These start from a slurry, in particular anaqueous solution, comprising the individual starting components of theelements in the appropriate proportions.

The starting materials of the individual components for preparing thecatalyst of the invention are, apart from the oxides, preferablywater-soluble substances such as ammonium salts, nitrates, sulfates,halides, hydroxides and salts of organic acids which can be convertedinto the corresponding oxides by heating. To mix the components, aqueoussolutions or suspensions of the metal salts are prepared and mixed.

In the case of molybdenum, it is advisable to use the correspondingmolybdates, e.g. ammonium molybdate, as starting compounds because oftheir commercial availability.

Suitable palladium compounds are, for example, palladium(II) chloride,palladium(II) sulfate, tetramminepalladium(II) nitrate, palladium(II)nitrate and palladium(II) acetylacetonate.

The reaction mixture obtained is then stirred at from 50 to 100° C. forfrom 5 minutes to 5 hours. The water is subsequently removed and thecatalyst which remains is dried at a temperature of from 50 to 150° C.,in particular from 80 to 120° C.

If the resulting catalyst is subsequently subjected to a furthercalcination process, it is advisable to calcine the dried and pulverizedcatalyst at a temperature in the range from 100 to 800° C., inparticular from 200 to 500° C., in the presence of nitrogen, oxygen oran oxygen-containing gas. The time is from 2 to 24 hours.

The catalyst can be used without a support material or can be mixed withor applied to an appropriate support material. Suitable supportmaterials are customary support materials such as porous silicondioxide, ignited silicon dioxide, kieselguhr, silica gel, porous ornonporous aluminum oxide, titanium dioxide, zirconium dioxide, thoriumdioxide, lanthanum oxide, magnesium oxide, calcium oxide, barium oxide,tin oxide, cerium dioxide, zinc oxide, boron oxide, boron nitride, boroncarbide, boron phosphate, zirconium phosphate, aluminum silicate,silicon nitride or silicon carbide or else glass, carbon fiber, metaloxide or metal meshes or corresponding monoliths.

Preferred support materials have a surface area of less than 100 m²/g.Preferred support materials are silicon dioxide and aluminum oxidehaving a low specific surface area. The catalyst can be used as aheterogeneous oxidation catalyst after shaping as regularly orirregularly shaped support bodies or else in powder form.

The reaction can be carried out in a fluidized bed or in a fixed-bedreactor. For use in a fluidized bed, the catalyst is milled to aparticle size in the ranqe from 10 to 200 μm.

The gaseous feed comprises ethane and/or ethylene which are fed to thereactor as pure gases or in admixture with one or more other gases.Examples of such additional or carrier gases are nitrogen, methane,carbon monoxide, carbon dioxide, air and/or water vapor. The gascomprising molecular oxygen can be air or a gas comprising more or lessmolecular oxygen than air, e.g. oxygen. The proportion of water vaporcan be in the range from 0 to 50% by volume. Higher water vaporconcentrations would make the work-up of the aqueous acetic acid formedunnecessarily expensive for process reasons. The ratio ofethane/ethylene to oxygen is advantageously in the range from 1:1 to10:1, preferably from 2:1 to 8:1. Relatively high oxygen contents arepreferred since the achievable ethane conversion and thus the yield ofacetic acid is higher. Oxygen or the gas comprising molecular oxygen ispreferably added in a concentration range outside the explosive limtisunder the reaction conditions since this makes the process easier tocarry out. However, it is also possible to employ an ethane/ethylene tooxygen ratio within the explosive limits.

The reaction is carried out at temperatures of from 200 to 500° C.,preferably from 200 to 400° C. The pressure can be atmospheric orsuperatmospheric, e.g. in the range from 1 to 50 bar, preferably from 1to 30 bar.

The reaction can be carried out in a fixed-bed or fluidized-bed reactor.

Advantageously, ethane is first mixed with the inert gases such asnitrogen or water vapor before oxygen or the gas comprising molecularoxygen is fed in. The mixed gases are preferably preheated to thereaction temperature in a preheating zone before the gas mixture isbrought into contact with the catalyst. Acetic acid is separated fromthe gas leaving the reactor by condensation. The remaining gases arerecirculated to the reactor inlet where oxygen or the gas comprisingmolecular oxygen and also ethane and/or ethylene are metered in.

On comparing the catalysts of the invention with those of the prior art,it is found that the present catalysts achieve higher space-time yieldsand acetic acid selectivities under identical reaction conditions(reaction feed gas, pressure, temperature).

When using the catalyst of the invention, the selectivity in theoxidation of ethane and/or ethylene to acetic acid is≧70 mol %,preferably≧80 mol %, in particular≧90 mol %, and the space-time yieldis>150 kg/hm³, in particular>200 kg/hm³, preferably>300 kg/hm³, so thatthe process of the invention enables, in comparison with the prior art,an increase in the acetic acid yields to be achieved in a simple mannerwhile simultaneously reducing the formation of undesired by-products.

EXAMPLES

The catalyst composition described in the examples is given in relativeatom ratios.

Catalyst Ppreparation

Catalyst (I):

A catalyst having the following composition was prepared:

Mo_(1.00)Pd_(0.00075)V_(0.45)Nb_(0.03)Sb_(0.01)Ca_(0.01)

Solution 1:

40 g of ammonium molybdate in 100 ml of water

Solution 2:

12.0 g of ammonium metavanadate in 200 ml of water

Solution 3:

4.19 g of niobium oxalate, 0.96 g of antimony oxalate, 0.68 g of calciumnitrate in 100 ml of water

Solution 4:

0.039 g of palladium acetate in 100 ml of acetone.

The aqueous solutions 1 to 3 are stirred separately at 70° C. for 15minutes. The third is then added to the second. The combined mixturesare stirred at 70° C. for 15 minutes before these are added to thefirst. Solution 4 is then added thereto. The resulting mixture isstirred at 70° C. for 15 minutes and subsequently evaporated to a volumeof 400 ml of solution. The mixture is spray-dried and calcined instationary air at 120° C. for 2 hours and 300° C. for 5 hours. Thecatalyst is then lightly ground in a mortar and pressed into pellets.These are crushed on a sieve to give a sieve fraction of from 0.35 to0.7 mm.

Catalyst (II):

A catalyst having the following composition was prepared:

Mo_(1.00)Pd_(0.0005)V_(0.45)Nb_(0.03)Sb_(0.01)Ca_(0.01)

Solution 1:

40 g of ammonium molybdate in 100 ml of water

Solution 2:

12.0 g of ammonium metavanadate in 200 ml of water

Solution 3:

4.19 g of niobium oxalate, 0.96 g of antimony oxalate, 0.68 g of calciumnitrate in 100 ml of water

Solution 4:

0.026 g of palladium acetate in 100 ml of acetone.

The Peparation of the Catalyst was Carried out as Described in CatalystExample (I).

Catalyst (III):

A catalyst having the following composition was prepared:

Mo_(1.00)Pd_(0.00085)V_(0.55)Nb_(0.06)Sb_(0.01)Ca_(0.01)

Solution 1:

40 g of ammonium molybdate in 100 ml of water

Solution 2:

14.7 g of ammonium metavanadate in 200 ml of water

Solution 3:

8.38 g of niobium oxalate, 0.96 g of antimony oxalate, 0.68 g of calciumnitrate in 100 ml of water

Solution 4:

0.044 g of palladium acetate in 100 ml of acetone.

The Preparation of the Catalyst was Carried out as Described in CatalystExample (I).

Catalyst (IV):

A catalyst having the following composition was prepared:

Mo_(1.00)Pd_(0.00075)V_(0.55)Nb_(0.09)Sb_(0.01)Ca_(0.01)

Solution 1:

40 g of ammonium molybdate in 100 ml of water

Solution 2:

14.7 g of ammonium metavanadate in 200 ml of water

Solution 3:

12.75 g of niobium oxalate, 0.96 g of antimony oxalate, 0.68 g ofcalcium nitrate in 100 ml of water.

Solution 4:

0.039 g of palladium acetate in 100 ml of acetone.

The Preparation of the Catalyst was Carried out as Described in CatalystExample (I).

Catalyst (V):

A catalyst having the following composition was prepared:

Mo_(1.00)Pd_(0.00085)V_(0.55)Nb_(0.09)Sb_(0.01)Ca_(0.01)

Solution 1:

40 g of ammonium molybdate in 100 ml of water

Solution 2:

14.7 g of ammonium metavanadate in 200 ml of water

Solution 3:

12.75 g of niobium oxalate, 0.96 g of antimony oxalate, 0.68 g ofcalcium nitrate in 100 ml of water

Solution 4:

0.044 g of palladium acetate in 100 ml of acetone.

The Preparation of the Catalyst was Carried out as Described in CatalystExample (I). Comparative Example

Catalyst (VI)

Catalyst Example (I) from the German Patent Application P 19630832.1,which is not a prior publication, having the compositionMo_(1.00)Pd_(0.0005)V_(0.25)Nb_(0.12) was prepared in order todemonstrate the higher space-time yield of the catalysts of theinvention.

Solution 1:

61.75 g of ammonium molybdate and 0.039 g of palladium acetate in 200 mlof water

Solution 2:

10.22 g of ammonium metavanadate in 250 ml of water.

Solution 3:

27.51 g of niobium oxalate in 25 ml of water.

The solutions are stirred separately at 90° C. for 15 minutes. The thirdsolution is then added to the second. The combined mixtures are stirredat 90° C. for 15 minutes before the first solution is added thereto. Theresulting mixture is stirred at 90° C. for 15 minutes. The mixture issubsequently spray-dried and calcined in stationary air at 120° C. for 2hours and 300° C. for 5 hours. The catalyst is then lightly ground in amortar and pressed into pellets. These are crushed on a sieve to give asieve fraction of from 0.35 to 0.7 mm.

Method of Catalyst Testing

A steel reactor having an internal diameter of 10 mm was charged with 5or 10 ml of the catalyst. The catalyst was heated to 250° C. in a streamof air. The pressure was subsequently adjusted by means of an admissionpressure regulator. The desired ethane:oxygen:nitrogen mixture was fedtogether with water into a vaporizer zone where water was vaporized andmixed with the gases. The reaction temperature was measured by means ofa thermocouple in the catalyst bed. The reaction gas was analyzedon-line by gas chromatography.

In the examples, the following terms are defined as:

 Ethane conversion(%)=100×([CO]/2+[CO₂]/2+[C₂H₄]+[CH₃COOH])/([CO]/₂+[CO₂]/2+[C₂H₂]+[C₂H₆]+[CH₃COOH])

Ethylene selectivity (%)=100×([C₂H₄])/([CO]/₂+[CO₂]/2+[C₂H₄]+[CH₃COOH])

Acetic acid selectivity(%)=100×([CH₃COOH])/([CO]/₂+[CO₂]/2+[C₂H₄]+[CH₃COOH])

where

[ ]=concentrations in mol % and

[C₂H₆]=concentration of the unreacted ethane.

The residence time is defined as:

t (s)=bed volume of the catalyst (ml)/volume flow of the gas through thereactor based on the reaction conditions (ml/s)

Reaction procedure

The reactor inlet gas consisted of 40% by volume of ethane, 8% by volumeof oxygen, 32% by volume of nitrogen and 20% by volume of water vapor.Since the space-time yield is dependent on the reaction pressure, allcomparative examples were carried out at 15 bar for reasons ofcomparability. Reaction conditions and results are summarized in thefollowing table.

Compared to the comparative catalyst (VI), significantly higherselectivities and space-time yields for acetic acid are achieved usingthe catalysts (I) to (V) at the same temperatures.

TABLE 1 Ethane Acetic acid Ethylene Space-time CO + CO₂ TemperaturePressure Residence conversion selectivity selectivity yield selectivityEx. Catalyst (° C.) (bar) time(s) (%) (%) (%) [kg/hm³] (%) 1 (I) 300 1514 15 63 25 184 11 2 (II) 300 15 14 15 69 19 193 12 3 (III) 300 15 10 1086 2 247 12 4 (IV) 280 15 15 14 91 1 215 8 5 (IV) 300 15 10 14 89 3 3558 6 (IV) 300 15 7 13 81 11 410 8 7 (IV) 310 15 7 15 85 3 470 12 8 (V)300 15 10 15 77 14 325 9 9 (VI) 280 15 30 15 77 2 72 21

Experimental Report Ethane Acetic acid Ethylene Space-time CO-CO₂Temperature Pressure Residence conversion select. select. yield select.Catalyst composition (° C.) (bar) time(s) (%) (%) (%) (kg/hm³) (%)Mo_(1.0)V_(0.55)Nb_(0.09)Bi_(0.03) 280 15 14.8 11.2 85.2 2.1 182 12.7Sb_(0.01)Ca_(0.01)Pd_(0.00075) Mo_(1.0)V_(0.55)Nb_(0.09) Bi_(0.03) 28015 7.4 6.4 76.9 6.3 187 16.8 Sb_(0.01)Ca_(0.01)Pd_(0.00075)Mo_(1.0)V_(0.55)Nb_(0.09) Bi_(0.03) 300 15 14.3 14.3 66.2 20.7 181 13.1Sb_(0.01)Ca_(0.01)Pd_(0.00075) Mo_(1.0)V_(0.55)Nb_(0.09) Bi_(0.03) 30015 7.1 10.6 47.9 37.6 193 14.5 Sb_(0.01)Ca_(0.01)Pd_(0.00075)Mo_(1.0)V_(0.55)Nb_(0.09)Sb_(0.03) 300 15 7.4 7.3 62.9 6.1 174 31.0Ca_(0.01)Pd_(0.00075)Au_(0.00028) Mo_(0.97)V_(0.55)Nb_(0.09)Sb_(0.01)280 15 14.8 13.1 83.0 3.2 206 13.9 Ca_(0.01)K_(0.01)Pd_(0.00075)Mo_(0.97)V_(0.55)Nb_(0.09)Sb_(0.01) 280 15 7.4 11.2 66.4 6.0 264 27.5Ca_(0.01)K_(0.01)Pd_(0.00075) Mo_(0.97)V_(0.55)Nb_(0.09)Sb_(0.01) 300 1514.3 15.0 68.9 19.2 196 11.9 Ca_(0.01)K_(0.01)Pd_(0.00075)Mo_(0.97)V_(0.55)Nb_(0.09)Sb_(0.01) 300 15 7.1 13.6 55.7 27.1 288 17.2Ca_(0.01)K_(0.01)Pd_(0.00075)

What is claimed is:
 1. A process for the selective preparation of aceticacid from a gaseous feed comprising ethane, or mixtures of ethane andethylene thereof plus oxygen at elevated temperature, which comprisesbringing the gaseous feed into contact with a catalyst comprising theelements Mo, Pd, X and Y in the gram atom ratios a:b:c:d in Combinationwith oxygen Mo_(a)Pd_(b)X_(c)Y_(d)  (I) where the symbols X and Y havethe following meanings: X is one or more elements selected from thegroup consisting of Nb, Ta, V, Te and W; Y is one or more elementsselected from the group consisting of Bi, Cu, Ag, Au, Li, K, Rb, Cs, Mg,Ca, Sr, Ba, Nb, Zr, Hf and Sb; the indices a, b, c, d and x are the gramatom ratios of the corresponding elements, where a=1; b=0.0001-0.01;c=0.4-1; and d=0.005-1 and the space-time yield in the oxidation toacetic acid is>150 kg/hm³, with the proviso that Mo and the elements Xdo not derive from a hetero-poly-acid.
 2. The process as claimed inclaim 1, wherein X and/or Y are a plurality of elements and the indicesc and d may assume different values for different elements.
 3. Theprocess as claimed in claim 1, wherein the temperature is in the rangefrom 200 to 500° C.
 4. The process as claimed in claim 1, wherein thepressure in the reactor is in the range from 1 to 50 bar (1.02-50.95kg/cm²).
 5. The process as claimed in claim 1, wherein b is in the rangefrom 0.0001 to 0.001.
 6. The process as claimed in claim 1, whereinethane mixed with at least one further gas is fed to the reactor.
 7. Theprocess as claimed in claim 6, wherein the further gas fed in isnitrogen, oxygen, methane, carbon monoxide, carbon dioxide, ethyleneand/or water vapor.
 8. The process as claimed in claims 1, wherein thecatalyst comprises at least one of the following compositions incombination with oxygen:Mo_(1.00)Pd_(0.0005)V_(0.45)Nb_(0.03)Sb_(0.01)Ca_(0.01)Mo_(1.00)Pd_(0.0005)V_(0.45)Nb_(0.03)Sb_(0.01)Ca_(0.01K) _(0.015)Mo_(1.00)Pd_(0.00075)V_(0.45)Nb_(0.03)Sb_(0.01)Ca_(0.01)Mo_(1.00)Pd_(0.00075)V_(0.55)Nb_(0.03)Sb_(0.01)Ca_(0.01)Mo_(1.00)Pd_(0.00075)V_(0.45)Nb_(0.06)Sb_(0.01)Ca_(0.01)Mo_(1.00)Pd_(0.0008)V_(0.55)Nb_(0.06)Sb_(0.01)Ca_(0.01)Mo_(1.00)Pd_(0.00085)V_(0.55)Nb_(0.06)Sb_(0.01)Ca_(0.01)Mo_(1.00)Pd_(0.00075)V_(0.55)Nb_(0.09)Sb_(0.01)Ca_(0.01)Mo_(1.00)Pd_(0.0008)V_(0.15)Te_(0.03)Sb_(0.01)Ca_(0.01)Mo_(1.00)Pd_(0.00075)V_(0.50)Nb_(0.09)W_(0.01)Pd_(0.01.)
 9. The processas claimed in claim 1, wherein the catalyst is mixed with a supportmaterial or fixed on a support material.
 10. The process as claimed inclaim 1, wherein the selectivity of the oxidation reaction of ethaneand/or ethylene to acetic acid is ≧70 mol %.